1. Field of the Invention
The present invention relates in general to an inductor device. In particular, the present invention relates to an inductor device for CMOS RF pre-stage circuits fabricated by semiconductor processes.
2. Description of the Related Art
Recently, CMOS technology has become more popular in circuit design, due to the integration of CMOS. When designing CMOS radio frequency circuits, control of inductor characteristics is very important.
FIG. 1 is a top view of a conventional planar inductor. FIG. 2 is a section view of a conventional planar inductor. The conventional planar inductor uses the top metal layer of a semiconductor device comprising coils in a predetermined direction. In the design of conventional planar inductors, the radius of the planar inductor should exceed 30 um and the loop count three to achieve the desired inductance for circuits. However, increasing the loop count of the inductor will cause an increase in the size of the inductor. Therefore, designers decrease the radius of the planar inductor to keep the size of conventional planar inductors smaller.
However, decreasing the radius of the planar inductor decreases inductance and lowers the quality of the inductor by intensifying the occurrence of eddy currents.
If the design increases the loop count of the inductor to increase inductance, eddy currents will be more pronounced, negatively impacting the quality of the inductor.
Moreover, conventional multi-layer inductor layout deposes multiple coils on one layer. If designers want to increase the inductance of the inductor, more coils must be deposed on other layers. This way will, however, increase the length of the current path and cause an increase in the voltage difference between the layers. Consequently, the capacitance of the inductor between the two layers increases, and the self-resonant frequency of the inductor is lowered.
The object of the present invention is to provide a multi-layer inductor to solve the problems mentioned above. The inductor according to the present invention exhibits a marked increase in the inductance to unit area ratio. Incidentally, the inductor of the present invention is smaller than conventional inductors of identical quality.
Moreover, inductance of the inductor according to the present invention is increased due to mutual induction between inner-portion coils and outer-portion coils. Therefore, the problem of low self-resonant frequency seen in conventional inductors is eliminated, and the size and cost of the RF circuit is decreased commensurately.
To achieve the above-mentioned object, the present invention provides an inductor device with both a first input/output terminal and a second input/output terminal. The inductor device comprises a first coil and a second coil. The first coil has a first inner coil and a first outer coil surrounding the first inner coil. The first inner coil has a first connect portion connecting to the first input/output terminal and a second connect portion. The first outer coil has a third connect portion and a fourth connect portion connecting to the second input/output terminal. The second coil location has a second inner coil and a second outer coil surrounding the second inner coil. The second inner coil has a fifth connect portion and a sixth connect portion connecting to the second connect portion. The second outer coil has a seventh connect portion connecting to the fifth connect portion and a eighth connect portion connecting to the third connect portion.